The present invention pertains to a pressure cell with a temperature sensor and a method for measuring pressure with such sensor. Temperature sensors are frequently integrated into pressure cells in order to compensate the static temperature error occurring during the pressure measurement. In ceramic pressure cells, the temperature sensor is usually applied onto the rear side of the ceramic. Alternatively, the temperature sensor may also be contained in downstream evaluation electronics.
If the pressure cell is in thermal equilibrium with its surroundings, the temperature dependence of the pressure measurement can be adequately compensated with such temperature sensor and suitable subsequent processing of the measuring signal. However, rapid changes in the temperature can lead to significant measuring errors that cannot be meaningfully compensated with known methods. This problem can arise in various types of pressure cells, but is particularly severe in ceramic measuring cells with flat membranes.
In order to solve this problem, the present invention proposes to arrange two temperature sensors in a pressure cell with a base body and a membrane that is arranged on the base body and can be deformed by the pressure to be measured such that they [said sensors] are spaced apart in the direction of an expected temperature gradient.
In most instances in which the pressure cell is not in thermal equilibrium, a difference in temperature can be expected between a medium acting upon the membrane of the measuring cell and the rear side of the pressure cell which faces away from the medium and is thermally connected to the surroundings, i.e., a temperature gradient from the front side to the rear side of the pressure cell results. Consequently, the first temperature sensor preferably is arranged on the membrane that forms the front side of the pressure cell where it is able to rapidly follow temperature changes of the medium, and the second temperature sensor is arranged on the rear side of the base body of the measuring cell which faces away from the membrane.
This proposition is based on the notion that the errors in the pressure measuring value caused by temperature changes can be traced back to an internal deformation of the measuring cell caused by a temperature gradient. If the membrane of the measuring cell is subjected to an abruptly heated medium, the membrane is able to follow this change in temperature significantly faster than the base body of the measuring cell which is spaced apart from the membrane by an intermediate space or a chamber and thermally insulated from the fluid. However, a rigid connection between the membrane and the base body prevents the membrane from expanding freely. This means that the thermal expansion causes the membrane to curve. This curvaturexe2x80x94either toward the base body or away from the base bodyxe2x80x94causes the errors in the pressure measurement. According to the invention, the determination of a temperature gradient makes it possible to recognize when the risk of a curvature of the membrane which could falsify the measuring value exists, and, if such a risk exists, to ignore the measuring values of the pressure cell or to compensate said measuring values by taking into consideration the membrane curvature.
A typical distance between the first temperature sensor and outer surface of the membrane which is subjected to the pressure to be measured lies between 0.1 and 3 mm depending on the thickness of the membrane, i.e., depending on the dimensions of the pressure cell and the intensity of the pressures to be measured.
In order to ensure efficient heat transfer between the medium and the first temperature sensor, the first temperature sensor is advantageously embedded in a material layer that connects the base body and the membrane.
It is particularly advantageous to embed the first temperature sensor in a seal that seals a chamber formed between the base body and the membrane.
In a pressure cell in which the base body and/or membrane consist(s) of a ceramic material, such a seal is advantageously manufactured from glass.
The respective temperature sensors preferably contain a resistance element with a temperature-dependent resistance value. Such a resistance element can be easily produced in a flat fashion such that it extends merely over a short distance in the direction of the expected temperature gradient.
In order to obtain a high-intensity temperature measuring signal with the least possible noise, it is practical that the resistance element of the first temperature sensor extend over essentially the entire circumference of the measuring cell. One side effect of this arrangement of the resistance element is that the temperature value determined from the resistance value of the resistance element represents an average value over essentially the entire circumference of the membrane such that this temperature value comes very close to an average value over the entire surface of the membrane.
In order to accommodate a long conductor length of the resistance element on a given circumferential length of the seal, the resistance element is preferably realized in a meander-shaped fashion.